1. Field of the Invention
This invention relates to a vertical filter circuit for a picture in picture (PIP) function and, more particularly, to a vertical filter circuit for constructing a sub-screen when the input signal of a main screen is a double-scanned signal or a HDTV signal.
2. Brief Discussion of Related Art
Generally, a PIP function is used for simultaneously displaying image signals of both the main screen and a sub-screen by horizontally and vertically compressing the sub-screen image signal. It will be appreciated that image signals of the same or different broadcasting channels may be displayed on the main and sub-screens. An image signal reproduced from an image recording/reproducing apparatus (VCR) be also displayed on the main screen, while the image signal of a broadcast channel can be displayed on the sub-screen with the size and position of the sub-screen being varied by a user.
The sub-screen for the PIP is processed as a digital signal. The data amount in the sub-screen of a predetermined size, i.e., a typical size ratio of sub-screen to main screen is 1 to 9, should be compressed into a predetermined amount corresponding to this ratio.
It will be noted that horizontal signal components of the sub-screen can be compressed into one third by setting the analog-to-digital (A/D) conversion clock frequency to one third of the digital-to-analog (D/A) conversion clock frequency. Moreover, vertical signal components of the sub-screen can be compressed into one third of the data amount of that of the main screen by reducing the number of scanning lines to one third. Accordingly, the data amount of the sub-screen can be compressed into one ninth of that of the main screen.
However, since a low sampling frequency is used for generating the abovedescribed sub screen, the video signal frequency must be restricted. Moreover, since the direction of the horizontal signal and the direction of the scanning line coincide with each other, the horizontal signal is continuous. To restrict the frequency band of the continuous signal, a low pass filtering must be performed before the A/D conversion. It will be noted that, when the vertical signal and the scanning line cross each other, the vertical signal becomes digitalized (discontinuous). The frequency band of this discontinuous signal cannot be restricted by present day analog technology. Therefore, to restrict the frequency band of the vertical signal, a digital filter called a vertical digital filter (hereinafter referred to as vertical filter) was designed. FIGS. 1 to 4F are useful in understanding the construction and operation of a typical vertical filter.
FIG. 1 illustrates a method for constructing the PIP sub-screen using a general interlace scanning signal. One line of PIP sub-screen data is generated for every third line of the image date per field.
A vertical filter is needed to remove or suppress a flicker phenomenon caused by a shortage in the number of vertical lines, i.e., so-called aliasing error. This is accomplished by restricting the frequency band of the discontinuous vertical signal. A high level diagram of the vertical filter is shown in FIG. 2, a more detailed example is shown in FIG. 3. This vertical filter technology used for providing PIP functions is described by Masashi Honzawa et al. in the article entitled "New Picture in Picture LSI Enhanced Functionality for High Picture Quality," in IEEE Transaction on Consumer Electronics, Vol. 36, No. 3, August, 1990.
Referring to FIG. 2, the vertical filter includes a three tap finite impulse response (FIR) filter adapted for vertical filtering and constructed using two 1H line memories 1 and 2, three multipliers 3, 4 and 5, and an adder 6. FIG. 3 illustrates a modified vertical filter. The modified filter uses only one 1H line memory 15 but performs the same function as the vertical filter circuit shown in FIG. 2. This is possible because only one significant line of the three lines of main screen data are necessary for compressing the sub-screen data.
The operation of the circuit shown in FIG. 3 will be described with respect to the waveforms shown in FIGS. 4A to 4F, wherein control signals CTL1 to CTL4 (shown in FIGS. 4B to 4E) are varied every third horizontal (H) period on the basis of the horizontal synchronous signal of FIG. 4A. These signals are supplied to AND gates 10, 11, 12 and 16 shown in FIG. 3. A final output signal of 1H line memory 15 can extract data (h.sub.2 Z.sup.-2 +h.sub.1 Z.sup.-1 +h.sub.0 Z.sup.0) every 3H period as shown in FIG. 4F.
The above-described vertical filter circuit can only be used for generating the PIP sub-screen when applied to a display device in which the main screen is generated by interlace scanning. When the input signal of main screen is a double-scanned signal or a HDTV signal, the number of scanning lines is increased by two, prohibiting use of the vertical filter.
U.S. Pat. No. 4,768,093 also discloses a vertical filter for PIP. This patent relates to a filter whose length is greater than a decimation factor in a television having a PIP function and which is applied to a general raster scan display device. However, both types of prior art filter cannot be applied to the double-scanned display device or HDTV.